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26 April 2019

Friday fold: Black Sands Beach

The Friday fold can be found in a boulder of gray chert layers on Black Sands Beach, at Marin Headlands in California.


12 January 2018

Friday folds: Hayden Butte (“A Mountain”), Tempe

In keeping with the Arizonarific theme of this week’s posts (thanks to my participation in the 2018 Structural Geology and Tectonics Forum), I thought I would wrap up my ‘geology of the Phoenix area‘ posts with a walk I took on my last day there. This was to what Google Maps calls “Hayden Butte,” but the locals call “A Mountain.” Not “a mountain,” but “the mountain called ‘A‘.” It has …


26 January 2017

Dore Holm

The scenic arch of Dore Holm (“Door Island”) in Shetland shows off the most efficient way of breaking a slab of rock. The island’s shape is a reflection of the parsimonious nature of natural deformation.


25 January 2017

Y-shaped joints on a basalt flow, Lake Mývatn, Iceland

A basalt flow in Iceland shows both enticing pahoehoe and fractures with a Y-shaped intersection pattern. Comparisons to bread loaves and east Africa suggest a reason why.


30 June 2016

Virtual field trip to Siccar Point, Scotland

Time for another virtual field trip on the Geologist’s Grand Tour of the United Kingdom: the most famous outcrop in the world. Today, we visit Siccar Point, Scotland. You’ve probably already seen photos of this place – they usually look something like this: To those who aren’t familiar, here’s what going on: There are two sets of strata here – and the contact between them is an ancient erosional surface. …


20 June 2016

Cushendun Conglomerate of the Cross Slieve Group, Northern Ireland

Want a geological irony? Here’s one! You’re looking at a rounded boulder of Cushendun Conglomerate, a Devonian “Old Red Sandstone” unit (Cross Slieve Group) exposed at Cushendun Caves, Northern Ireland, U.K. The irony lies in the repetition of history – a tumbling environment of high water energy, rounding cobbles and boulders and depositing them, in order to make the conglomerate. And now, ~400 million years later, history repeats itself, with …


13 January 2015

Glacial striations and robust hackles in Jasper

Check out the argillite boulder in the left midground of this GigaPan, which I’ve showed here before. It was taken at the Icefields Center parking area in Jasper National Park, Alberta: [gigapan id=”159489″] link There, you’ll find some lovely orange lichens, some iron oxide staining, some graffiti, and a fair number of sub-aligned glacial striations. Also, at the top edge of the boulder, there’s a nice set of big hackles, …


7 May 2013

Brallier Formation 2: tectonic structures

Yesterday we examined primary sedimentary structures (including trace fossils) at an outcrop of Devonian-aged Brallier Formation turbidites between Deerfield and West Augusta, Virginia. Today, we’ll zoom in on the tectonic structures at the site: folds, faults, and joints. Remember, you don’t have to take my word for it. You can explore it for yourself in this M.A.G.I.C. GigaPan: [gigapan id=”128612″] link One thing that’s kind of cool about that GigaPan …


12 March 2013

Five new GigaPans from Thoroughfare Gap

Yesterday, I took five new GigaPans at Thoroughfare Gap, a water gap where Broad Run cuts through Bull Run Mountain, the eastern limb of the Blue Ridge Anticlinorium at my latitude. The rocks here are the Cambrian-aged Chilhowee Group, with bedding tilted moderately to the east during Alleghanian mountain-building in the late Paleozoic. To the west is the crystalline core of this massive regional fold, and to the east is …


1 March 2013

Friday fold: An odd duck

A USGS colleague shows Callan a bizarre fold outcrop in the Conococheague limestone of the Boyce quadrangle, Virginia.


24 November 2012

Rock Cycle III: Igneous → Sedimentary

The Boulder Batholith outside of Butte, Montana, is actively weathering, and shedding off grus. In the third installment of the Transitions of the Rock Cycle series, we watch an igneous rock turn to sediment.


21 November 2011

Compton Peak columns

Callan and three students visit a world-class outcrop of columnar jointing in Shenandoah National Park.


25 July 2011

Swift Dam

Dave Lageson of Montana State University and I ran into each on our respective field trips sometime last year outside of the Two Medicine Dinosaur Center in Bynum, Montana, and he told me that I had to go check out the Swift Dam area, a little ways northwest of the town of Depuyer. So, a year later, finding myself in Depuyer (as one does), I finally did. One thing I …


24 August 2010

Tipping your tension gash

Tension gashes are small veins that open up when rocks get stretched. Often, they are arrayed en echelon with respect to other tension gashes, all oriented in the same direction. Here is a sample of tension gashes I found this summer in rip-rap (i.e., not in situ) at some building site in New England. (I forget where, but it doesn’t matter, since it’s rip-rap. Could have come from anywhere!) Check …


20 August 2010

The Purgatory Conglomerate

After my thesis defense at the University of Maryland, my mentor and friend E-an Zen asked me if I had ever heard of the Purgatory Conglomerate. I had not. Over the years, E-an has been a great source of new ideas and information to me, and so when he raises a notion, I pay attention. In my thesis, I had done some strain analysis on volcanic clasts in a meta-ignimbrite …


20 May 2010

Falls of the James II: fractures

In my previous post, I introduced you to the Petersburg Granite, as it is exposed south of Belle Isle, at the falls of the James River in Richmond, Virginia. I mentioned that it was fractured, and I’d like to take a closer look at those fractures today. The geologically-imparted fractures were exploited by human granite quarriers, and in some parts of the river bed, you can see the holes they …